Microporous/non-porous composite membranes

ABSTRACT

A microporous/non-porous membrane laminate construction is described for incorporation in membrane package construction. One or both sides of the non-porous membrane is covered with a microporous membrane to form a membrane assembly. An exemplary membrane package construction employing the membrane assembly is described.

United States Patent Neulander et a1. 4

[451 Mar. 19, 1974 1 MICROPOROUS/NON-POROUS COMPOSITE MEMBRANES [75]Inventors: Charles K. Neulander; William J.

Ward, III, both of Schenectady, NY.

[73] Assignee: General Electric Company,

' Schenectady, NY.

[22] Filed: Aug. 27, 1971 21 Appl. No.: 175,670

[52] US. Cl. 55/158 [51] Int. Cl B0ld 53/22 [58] Field of Search 55/16,158; 210/321, 500

[56] References Cited UNITED STATES PATENTS 3,520,803 7/1970 laconelli55/16 3,624,983 12/1971 Ward 55/16 2,960,462 11/1960 Lee et a1 210/3213,447,286 6/1969 Dounoucos 55/16 3,416,985 12/1968 Dounoucos 55/163,510,387 5/1970 Robb 55/16 3,567,810 3/1971 Baker 264/41 3,615,02410/1971 Michaels 210/490 Primary Examiner-Charles N. Hart Attorney,Agent, or Firm-Leo I. MaLossi; Joseph T. Cohen; Jerome C. Squillaro [57] ABSTRACT A microporous/non-porous membrane laminate construction isdescribed for incorporation in membrane package construction. One orboth sides of the nonporous membrane is covered with a microporousmembrane, to form a membrane assembly. An exemplary membrane packageconstruction employing the membrane assembly is described.

5 Claims, 8 Drawing Figures PATENTED "AR I 9 I974 SHEET BF 4 FIG. 6

H/GH PRESSURE SIDE ' LOW PRESSURE S/DE /A/ l/E/V TORS CHARLES K.NEULA/VDER;

VIZ/LL/AM J. WARD ZZT,

' www THE /R A T TOR/VE Y MICROPOROUS/NONPOROUS COMPOSITE MEMBRANESBACKGROUND OF THE INVENTION Exemplary membrane package constructions areshown in US Pat. Nos. 3,564,819 Neulander et al.; 3,354,618 Dounoucosand 3,416,985 Dounoucos. The Neulander et al. patent is incorporated byreference.

In each of these constructions a plurality of spaced single membranesare disposed in substantially parallel surface-to-surface array so as todefine both a first group of flow volumes and a second group of flowvolumes alternating between the first group. After assembly, the edgesof the membranes and spacing means are bonded together to form a sealeddevice.

The term non-porous as applied herein refers both to solid imperforatemembranes and to porous membranes containing liquid lodged in the poresthereof.

It has been found in testing completed gas separation membrane packagesusing non-porous membranes, that an unacceptably large number ofmembrane packages had to be rejected because of cross-membrane leakage.This leakage became evident under the application of pressuredifferentials across the membranes during testing. These leaks appearedto be the result of very tiny pin holes or tears in the membranestructure, which either were present before assembly of the package andwere not visually detectable, or were caused after assembly of themembranes to form the package. The damage caused in the latter instancecould have resulted (a) from compression of the membranes into adjacentscreen separators or else (b) from abrasion of the membranes by theadjacent screen separators. The leak rates for such cross-membranebreaches are of the order of at least several cubic centimeters/sec. forpres sure differences as small as 1-2 psi. Such leakage rates are, ofcourse, unacceptable for most gas separation applications.

SUMMARY OF THE INVENTION The instant invention greatly reducescross-membane gas leakage by employing in place of each of the spacedsingle membranes a membrane assembly comprising a non-porous membraneand at. least one microporous membrane in face-to-face contacttherewith. If only one microporous membrane is employed it is disposedon what will be the high pressure side of the non-porous membrane duringuse. The microporous membrane is usually thinner and more wear resistantthan the nonporous membrane and, in view of its disposition on the highpressure side of the non-porous membrane, will automatically enter andplug any breaches therethrough.

BRIEF DESCRIPTION OF THE DRAWINGS The exact nature of this invention aswell as objects and advantages thereof will be readily apparent fromconsideration of the following specification relating to the annexeddrawing in which:

FIG. 1 is an exploded view of the spacing means and interposed membraneassembly constructions and shows the spacing means as they would appearin the completed structure (the end plates and gaskets are not shown);

FIG. 2 is an enlarged sectional view taken on line 2-2 of FIG. 8 (therelationship of this section to each disposition of the spacing meansencountered in the device is also shown in FIG. 1);

FIGS. 3, 4 and 5 are similar to FIG. I being taken on lines 33, 4-4 and5-5, respectively;

FIG. 6 is an enlarged sectional view of a membrane assembly according tothis invention in which a single microporous membrane is employed incombination with a non-porous membrane;

FIG. 7 is an enlarged sectional view of a membrane assembly wherein amicroporous membrane is disposed against each major face of a non-porousmembrane,

FIG. 8 is a three-dimensional view of a membrane package in which theinstant invention may be employed.

DESCRIPTION OF THE PREFERRED EMBODIMENT Although the improvedconstruction of the instant invention may be applied to gas separationdevices, gas concentrator devices, blood oxygenators, etc., theutilization thereof for the preparation of gas separation devices willbe described by way of example.

A repetitive stack arrangement comprising separator screen 11, membraneassembly 12, separator screen 11a, and the next membrane assembly 12 isshown in FIG. 1. Each membrane assembly 12 comprises nonporous membrane12a and microporous membrane 12b. The particular disposition ofmembranes 12a and 12b relative to each other is such as to placemicroporous membrane 12b on the high pressure side of nonporous membrane12a during operation. Separator screens 11, 11a are identically made,but are disposed in reverse fashion in the stack. I

The microporous membranes useful in this invention have pore sizesranging from about 30 to about 3,000A and have porosities (pore volumes)ranging from about 30 to about percent. The gas permeability ofmicroporous membranes useful in this invention should be in the range offrom about 10 to about times that of the particular non-porous membraneemployed in order to avoid significant resistance to gas penetrationtherethrough. The ratio of the thickness of the microporous membrane tothe thickness of the non-porous membrane may be in the range of fromabout 1:1 to 1:10. The preferred range of ratios is 1:4 1:6. Further,the microporous membrane material should be abrasion resistant andflexible.

Among the suitable materials available in microporous form arepolypropylene, polyethylene and polytetrafluoroethylene. In preparingmembrane assemblies involving immobilized liquid membranes, themicroporous membrane must be of a material that is not wet by the liquidconstituent of the immobilized liquid membrane.

Preferably, separator screens 11, 11a are woven screens e.g., polyesteror nylon monofilament screen cloth having mesh openings in the range ofabout 400 microns to about 1,200 microns and an open area ranging fromabout 45 to 55 percent. Other sizes of mesh openings and percentages ofopen area may be used depending upon the strength of the membranesemployed, the pressure difference prevailing across the membrane and thecriteria to be set for fluid flow through the screens. Screens preparedfrom other materials may be employed so long as the material is capableof retaining its structural integrity under the operating conditions andis compatible with the fluids employed.

The preparation of various imperforate membranes for gas separation isdisclosed in U.S. Pat. No. 3,396,510 Ward et al.; U.S. Pat. No.3,335,545 Robb et al., and U.S. Pat. No. 3,325,330 Robb, for

example. Useful capability for resisting pressure differentials has alsobeen obtained with immobilized liquid membranes in which the liquid islodged in the pores of a microporous layer made of a material wet by theliquid used.

By way of example, an imperforate immobilized liquid membrane wasprepared by impregnating a 5 mil thick sheet of the filter material soldunder the trademark Solvinert" (0.25 micron nominal pore size) with asaturated cesium bicarbonate aqueous solution. Tests have shown that thebubble point (the pressure needed to dislodge the liquid from the pores)for this material is at least 30 psi, if the Solvinert is soaked incesium bicarbonate solution. This material (Solvinert) appears underinfrared analysis to be terpolymer of polyvinyl alcohol, polyvinylchloride and polyvinyl acetate.

Other membrane materials useful as immobilizing mediums for variousliquids wet thereby include the ultrafine porous membrane disclosed inU.S. Pat. application Ser. No. 717,893 Weininger et a1. (filed Apr. 1,1968, now abandoned, and assigned to the assignee of the instantinvention) and the materials described in U.S. Pat. Nos. 3,378,057Sargent et al.; 2,984,869 Honey et al.; 3,351,489 Larson et al.;3,216,882 Feldt et al., and 3,062,760 Dermody et al. The disclosures ofthe aforementioned patent application and patents are incorporatedherein by reference in their entirety.

Before interleaving the membrane assemblies 12 between separator screensll, 11a the separator screens must be prepared so that after the stepsthat follow (assembly and potting) have been executed and the pottingmedium has hardened, the manifolding system may be developed simply bydrilling holes through the package.

The preparation referred to is the impregnating of each separator screenwith peripheral bead segments 13 and 14 and inwardly-directed beadextensions 17 as shown. The beads should be of uniform height relativeto the plane of the screen. Preferably, the material of which the beadsare composed will be a chemically inert self-setting liquid, which setsor cures as an elastomeric material. As may be seen from the drawing,the bead extensions 17 partially define small spaced areas, which fallinto two categories, (a) those which are additionally defined by beadsegments 13, 14 (note bead portions 130, 14a) and (b) those betweenwhich bead material is missing. Separator screen 11 differs fromseparator screen 11a only in its orientation in the assembly as may beseen in FIG. 1. Because of this difference in orientation and repetitionof the two orientations, each of the two different categories of definedsmall spaced areas will be aligned with similar area of the samecategory, when screen separators and membranes are assembled. Thepurpose for the presence or absence of bead portions 13a, 14a isexplained hereinbelow.

The preparation of membrance assembly 12 will depend upon whether themicroporous membrane is made of material that will be readily bonded tothe nonporous membrane by an intermediate layer of the potting compoundsas described hereinbelow. If the microporous membrane material is notreadily bonded to the non-porous membrane 12a by the particular pottingcompound selected, it will be necessary to treat those discrete areas ofthe microporous membrane (both sides) in order to make these surfaceswetting and bondable.

These discrete areas of membrane will be those areas that will becontiguous (when assembled) with the areas of screen's 11, 11a lyingbetween extensions 17 and along the border areas thereof. Themicroporous sheet can be masked so as to treat only the particular areasrequired for bonding and treatment may involve exposure to coronadischarge or chemical treatment to make these areas bondable. In thecase of polypropylene, oxidation of the surface areas to be treated(e.g., by corona discharge or by chemical oxidation) has beensuccessfully employed to render the polypropylene bondable.

In any case (whether or not microporous membrane 12b has requiredtreatment) microporous membrane 12 is attached to the non-porousmembrane 12a by applying a thin film of the potting compound over thetreated areas thereby securing the membranes into a single unit. Theseunits are then allowed to cure, after which they can be handled as asingle membrane.

When the desired number of bead-impregnated separator screens 11, 11ahave been prepared, the membrane assemblies 12 and separating screens11, are stacked so that each pair of membrane assemblies 12 has aseparator screen therebetween. However, the disposition of the screenseparators above and below any given membrane differs in that beadportions 13a, 14a in any given screen separator will overlie locationsin the separator screen immediately therebelow from which bead portionsare missing. Bead portions of alternate separator screens will be invertical alignment and similarly bead portions 14a of alternateseparator screens will be in vertical alignment.

When the package of membrane assemblies and separating screens has beenmade, the assembly is clamped tight. Thereafter, each edge of theassembly is immersed to a predetermined depth in a liquid pottingcompound. The liquid potting compound enters the package through theexposed edges of the separator screens until it encounters beads 13 and14, which prevents its further penetration, except for each defined areabetween bead extremities 17, where a gap exists between beads 13 and 14.In these areas and along the edge regions the potting compound is freeto enter to the extent shown (stippled area).

An example of a suitable potting compound for use with Solvinertmembranes in which a liquid membrane has been immobilized is abisphenol-A based epoxy resin to which has been added a catalyst systemconsisting of a modified polyamine plus polyamide. Another usefulpotting material is polyester adhesive.

The bead material is preferably an elastomer such as room temperaturevulcanizing silicone rubber, although other materials such as wax orputty may be satisfactorily employed for this damming function. In thecase of a potting liquid curing as an elastomer it is preferred to applythe bead material to the screen and then place the screen between platesurfaces (e.g., plexiglass) with spacing means such that the bead whencured (plates removed) is slightly thicker than the screen itimpregnates and is of uniform height from the plane of the screen. Thuswhen in the assembly (FIGS. 2-5) the beads are under compression and arein sealing engagement with adjacent surfaces of the membrane assembly 12during the potting.

After completion of the potting operation and subsequent curing, theedges of each membrane and separator screen in the assembly are bondedinto unified walls such as is shown in FIGS. 2-5. Wherever portions ofthe damming beads 13, 14 are lacking, the potting liquid proceedsfurther inward to develop projections, or extensions, 18 in verticalalignment over each other (in alternate layers).

When the potting material has hardened, holes are drilled through thepackage transverse to the direction of the planes of the laminae. Eachof these holes passes through each screen and through each extension 18that is encountered to form a vertical manifolding arrangement.

Thus, in the assembled membrane package 19 the same sequence of holesexists along a section taken on line 2-2 (FIG. 2) as occurs along asection taken on line 3 3 (FIG. 3). Similarly, the same sequence ofholes exists along a section taken on line 4-4 (FIG. 4) as occurs alonga section taken on line 55 (FIG. 5). By way of illustration, in FIGS. 2and 3 hole 21 is the hole through the uppermost separator screen andthis hole passes through screen material only. Hole 22 is the holethrough the uppermost membrane assembly 12. Hole 23 is the holeimmediately thereunder and this hole passes through both the pottingmaterial of extension 18 and the screen embedded therein. The samesequence then repeats itself. In the sections shown in FIGS. 4 and 5,the sequence of holes begins with a hole 23 through the uppermostscreen, passing through both the potting material of extension 18 andthe screen embedded therein.

The membrane package 19 is then sandwiched between metal end plates 24,26 and gaskets 24a, 26a having holes 27, 27a, respectively, matchingwith the vertical sequence of manifolding holes in the membrane package19. End plates 24, 26 serve both to force the gaskets into sealingengagement with the membrane package 19 and to provide terminals forconduits 28, 29, 31, 32 as shown in FIG. 8. Fasteners 33 arranged aroundthe perimeter of the device as shown in FIG. 8 provide biasing force forthe end plates.

Optionally, the first and last laminae of the membrane package 19 may beof solid sheet, e.g., thin sheet metal (pre-drilled for manifoldconnections) to provide stiffness and protection for the package. Whenemployed, such stiffener sheets become an integral part of the packageafter the potting operation. End plates and gaskets are then employed asdescribed hereinabove.

In operation a flow of feed gas entering through conduit 28 will exitvia holes 21 to pass through the alternate flow volumes A,, A Acomprising flow volume group A. This feed gas stream sweeps over thesurfaces of microporous membrane(s) 12b forming the boundaries thereofon its way to exit conduit 29 via holes 21 at that vertical manifold asis shown in FIG. 3. Simultaneously, the sweep gas stream enters conduit31 and is manifolded via holes 21 into flow channels B B B comprisingflow volume group B. The sweep gas stream exits via holes 21 from flowvolume group B to leave the device via conduit 32 as is shown in FIG. 5.

Usually, the feed gas enters under pressure significantly greater thanthe sweep gas. As a result, the pressure in flow volumes A A A isgreater than the pressure in the adjoining flow volumes constitutinggroup B. For this reason, the membrane assemblies 12 are disposed sothat microporous membranes 12b face the flow volumes of group A (thehigh pressure side) and, should a tear or other breach occur in any ofmembranes 12a, the entire pressure load at such discontinuities ismaintained by the microporous membrane 12b. This force is sufficient topush the microporous membrane 12b into the breach (FIG. 6) sealingagainst the rim thereof. This sealing plug, in-effect, presents a smallarea of microporous membrane 12b in place of the open breach 40. As aresult the cross-membrane leak rates are very drastically reduced,reducing in turn the reject rate of membrane packages. By way ofexample, the cross-membrane leak rates for small pin holes or minortears have been observed to be of the order of at least 3 cubiccentimeters/sec for pressure differences ranging from 1 to 2 psi. Incontrast thereto membrane assemblies 12 in which a one mil microporouspolypropylene membrane (Celanese Corp.) was placed on the high pressureside of a 5 mil thick immobilized liquid membrane that had been piercedwith a needle and/or slit with a razor, leak rates of the order of 0.1cc/sec. were measured on the low pressure side for pressure differencesup to 20 psig.

A one-foot square immobilized liquid membrane was pierced and slit andincorporated into a threemembrane test module as an assembly with twomicroporous polypropylene membranes (each 1 mil thick), one disposed oneach side of the damaged immobilizedliquid membrane as shown in FIG. 7.At crossmembrane pressures of 10 psi, the maximum leak rate observedthrough discontinuities such as hole 41 was about 0.67 cc/sec. Thus, theeffectiveness of the microporous layer 12b to seal leaks in non-porousmembranes 12a is due to the large permeability thereof compared to thepermeability of the non-porous material, but the small permeabilitythereof compared to the permeability of the breach. For example, in thecase of an immobilized cesium bicarbonate membrane 5 mils thick for CO/O separation, the microporous film can be of a thickness in the rangeof from about 1 to about 5 mils and have a C0 permeability ranging fromabout 5,000 X 10 to 50,000'X 10"? The construction shown in FIG. 7 is ofparticular advantage not only for the healing capability fordiscontinuities in the non-porous membrane 12a, but also for thecapability offered for protecting such membranes 12a (which aretypically easily damaged) from injury by contact with spacer screens 11,11a.

At each channel (hole 23) provided through an extension 18, the fluidflow traversing the channel is prevented from entering the flow channelabutting the extension by the presence of the potting materialcomprising extension 18 and forming the confining walls of each hole 23.In addition, these confining walls serve to bond together the screenseparator embedded therein and each membrane assembly 12 located at aface of extension 18. As described hereinabove, membranes 12a and 12b ineach assembly 12 will have been previously bonded together in these sameregions and along the borders.

The membrane assembly constructions of the instant invention may beemployed in any gas transfer membrane package construction wherein asheet of nonporous membrane forms at least part of the wall area of aclosed volume and means are provided to support the membrane (or toseparate adjacent membranes) whereby the membrane can be subjected to apressure differential.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. In a packaged membrane system in which (a) a plurality of spacednon-porous membranes are disposed in surface-to-surface array to defineat least a first closed flow volume and at least a second closed flowvolume adjacent thereto, (b) spacing means for the membranes is locatedtherebetween, and (c) means are provided for separate access to andegress from said first and second flow volumes, the combination witheach of said non-porous membranes of at least one microporous membranein face-to-face contact therewith, said microporous membrane having agas permeability of at least about 10 times the gas permeability of saidnon-porous membrane, the ratio of the thickness of said microporousmembrane to the thickness of said non-porous membrane being in the rangeof from about 1:1 to about 1:10.

2. The combination of claim 1 wherein the nonporous and microporousmembranes are bonded together over part of their contact surface area.

3. The combination of claim 1 wherein the nonporous membrane is animmobilized liquid membrane.

4. The combination of claim 1 wherein the microporous membrane is madeof polypropylene.

5. The combination of claim 1 wherein one microporous membrane isdisposed on each side of the nonporous membrane.

2. The combination of claim 1 wherein the non-porous and microporousmembranes are bonded together over part of their contact surface area.3. The combination of claim 1 wherein the non-porous membrane is animmobilized liquid membrane.
 4. The combination of claim 1 wherein themicroporous membrane is made of polypropylene.
 5. The combination ofclaim 1 wherein one microporous membrane is disposed on each side of thenon-porous membrane.